Direct-fired nitrogen vaporizers have been used, for example, to supply nitrogen for various oil and gas production operations. Such vaporizers commonly burn diesel or similar liquid fuels. In a typical direct-fired nitrogen vaporizer, the hot combustion gas generated by burning the liquid fuel is used to heat and vaporize a high-pressure liquid nitrogen stream.
A typical direct-fired nitrogen vaporizer will include: a burner barrel wherein the liquid fuel is combined with air and is burned; a plurality of burner assemblies positioned in an end wall of the burner barrel; a fan which supplies air to the burner barrel; and a nitrogen tube assembly through which a high-pressure liquid nitrogen stream flows. The high-pressure liquid nitrogen stream flowing through the tube assembly is heated and vaporized by the hot combustion gas flowing from the burner barrel. Each of the burners used in the burner barrel will typically include at least one liquid spray nozzle which is operable for spraying the liquid fuel into the burner barrel and a plurality of air slots extending radially about the periphery of the spray nozzle. The air fan operates to blow air through these air slots and into the burner barrel. The flow of air from the air fan also forces the combustion gas generated in the burner barrel out of the burner barrel and through the nitrogen tube assembly.
The liquid fuel supplied to the burner barrel is ignited using a pilot burner assembly. A pilot burner assembly 2 of the type used heretofore in the art is depicted in FIGS. 1 and 2. Pilot burner assembly 2 is composed of: a combustion chamber assembly 4 having a small diameter cylindrical chamber 6 and a large diameter cylindrical chamber 8; a nozzle holder 10 positionable in small diameter chamber 6 of assembly 4; and a diesel fuel nozzle 12 which is positionable in nozzle holder 10 such that, when nozzle holder 10 is placed in small diameter chamber 6, nozzle 12 is operable for atomizing and spraying diesel fuel into large diameter chamber 8 of assembly 4.
As shown in FIGS. 3-9, nozzle holder 10 is formed from a cylindrical piece having side portions 14, 16, and 18 removed therefrom. When nozzle holder 10 is positioned in small diameter chamber 6 of assembly 4, removed portions 14, 16, and 18 allow air from the vaporizer air fan to flow through small diameter chamber 6 and into the large diameter chamber 8 of assembly 4.
Nozzle holder 10 includes: a diesel fuel passageway 20; an ignition chamber 24 wherein a combustible gas (e.g., propane) is ignited; a combustible gas passageway 26 which communicates with ignition chamber 24; and a threaded spark plug port 28 which communicates with ignition chamber 24. Diesel fuel passageway 20 is completely separate from ignition chamber 24, combustible gas passageway 26, and threaded spark plug port 28. Diesel fuel passageway 20 includes (a) a threaded inlet port 30 for threadedly receiving a diesel fuel supply connection 32 and (b) a threaded outlet port 34 for threadedly receiving diesel fuel nozzle 12. A spark plug 36 is threadedly receivable in spark plug port 28 such that the sparking elements of spark plug 36 extend into ignition chamber 24. Combustible gas passageway 26 includes a threaded inlet port 38; a cylindrical bore 40 adjacent inlet port 38; a tapered threaded bore 42 adjacent cylindrical bore 40; and a small diameter cylindrical bore 44 extending from threaded bore 42 to the inlet 46 of ignition chamber 24. The inlet 46 of ignition chamber 24 has an inverted frusto-conical shape which diverges away from small diameter cylindrical bore 44. The remainder of ignition chamber 24 has a substantially cylindrical shape.
A gas nozzle 50 is threadedly receivable in the inlet port 38 of gas passageway 26 for delivering a combustible gas to passageway 26 and to ignition chamber 24. An orifice 52 for metering the rate of gas flow into ignition chamber 24 is threadedly received in small diameter bore 44 of gas passageway 26.
Combustion chamber assembly 4 is connectable to the end wall of a nitrogen vaporizer burner chamber using bolts 54. An aperture 56 is provided in the wall of small diameter cylindrical chamber 6 of assembly 4 for receiving spark plug 36 and allowing spark plug 36 to be threadedly connected in spark plug port 28 of nozzle holder 10. When positioned in aperture 56 and in threaded spark plug port 28, spark plug 36 operates to prevent nozzle holder 10 from falling or being pushed from small diameter chamber 6 into large diameter chamber 8 of combustion chamber assembly 4.
In operation, propane or some other combustible gas is delivered to ignition chamber 24 of pilot burner assembly 2 via gas nozzle 50, gas passageway 26, and orifice 52. In ignition chamber 24, the combustible gas is combined with air and is ignited by spark plug 36. The air needed for ignition in ignition chamber 24 enters the ignition chamber via ignition chamber opening 58. Air enters opening 58 as a result of turbulence created in the vaporizer burner barrel due to the operation of the vaporizer air fan.
At the same time that the air/combustible gas mixture is being ignited in ignition chamber 24, diesel fuel is delivered to large diameter cylindrical chamber 8 of assembly 4 via diesel supply connection 32, diesel fuel passageway 20, and diesel fuel nozzle 12. As the diesel fuel is discharged from diesel fuel nozzle 12, it is typically atomized and sprayed into chamber 8 in a solid or hollow cone pattern.
Upon being ignited in ignition chamber 24, the gas mixture in ignition chamber 24 flows out of ignition chamber opening 58 and thereby contacts and ignites the diesel fuel being sprayed into chamber 8 via fuel nozzle 12. The resulting flame generated in chamber 8 operates to ignite the primary diesel fuel burners associated with the vaporizer burner barrel. Once the diesel burner portion of pilot burner assembly 2 is ignited, the flow of combustible gas to ignition chamber 24 is typically discontinued.
Unfortunately, prior art pilot burner assembly 2 will typically require frequent maintenance as a result of serious soot accumulation problems. In order to allow the combustible gas material used in pilot burner assembly 2 to be ignited, the gas ignition portion of burner assembly 2 is designed and positioned such that low speed turbulent air flow is maintained in ignition chamber 24. However, as a result of this design, undesirable swirling and/or vortexing air flow patterns are generated in the gas ignition area such that a substantial amount of soot accumulates in the gas ignition area. Such soot accumulation can quickly render the gas ignition system inoperable.
A substantial amount of soot accumulation will also typically occur within large diameter combustion chamber 8 and around diesel fuel nozzle 12. This soot accumulation apparently results from the presence of dead spaces within chamber 8 wherein undesirable swirling and/or vortexing flow patterns are generated.
In using pilot burner assembly 2, difficulties are also typically encountered when attempting to ignite the pilot burner at low vaporizer air fan speeds. A low fan speed will typically yield a low air turbulence level in chamber 8 such that the rate of air entry into ignition chamber 24 is relatively low. Consequently, an excessively fuel-rich mixture is produced in ignition chamber 24.
Additional difficulties have been encountered in keeping the prior art pilot burner lit at high air fan speeds. High air fan speeds are used when the vaporizer system is required to supply a high vaporized nitrogen product rate. However, high air turbulence levels produced in combustion chamber 8 at high fan speeds can produce excessively fuel lean combustible gas/air mixtures in ignition chamber 24. High localized turbulent gas speeds in chamber 8 can also operate to blow out the pilot burner flame.
Further problems have been encountered in the use of the prior art burner as a result of the conically-shaped burner flame impinging upon, and thereby burning out, portions of cylindrical combustion chamber 8.